Unlicensed mobile access generally describes the accessing of public land mobile networks using access networks that utilise a typically low-power unlicensed-radio interface to communicate with mobile stations. Existing access networks typically include broadband networks that may include both wireless and wired portions, preferably IP networks, such as wireless LANs, in which higher layer protocols, for example, the GSM protocols, are run over an IP network rather than over the associated GSM radio layer. An access network controller communicates over the IP network with mobile stations that are connected to the network via access points. The access network controller also controls the interface with the public mobile network core elements that provide specific services to the mobile stations depending on the type of the public mobile core network. A security gateway also forms part of the access network and is either combined with the access network controller in a single node or implemented as a separate node. Access networks of this type have been used to provide access to second generation PLMNs including GSM (Global System for Mobile Communication), EDGE (Enhanced Data rates for GSM Evolution) and GPRS (General Packet Radio Service networks). In this context, unlicensed access networks are referred to as generic access networks (GAN) and the access network controller is called a generic access network controller or GANC, which includes a microprocessor connected with a non-transient memory containing instructions for carrying out the operations of the GANC.
Work is now ongoing in 3GPP for Release 8 to specify generic access networks for third generation services UMTS (Universal Mobile Telecommunications System) or WCDMA (Wideband Code Division Multiple Access). The corresponding technical specifications will be called 3GPP TS 43.319 and 44.319 and will also include the previous content from specifications 43.318 and 44.318 in 3GPP Releases 6 and 7. The generic access network is essentially transparent when viewed from the PLMN core network nodes. This is achieved by the GANC utilising the standard PLMN interfaces towards the various nodes. The existing 3GPP Releases 6 and 7 define how second generation mobile services are supported in the GANC and in a PLMN where access is provided by a GANC. Second generation (2G) mobile services are generally understood to comprise GSM (Global System for Mobile Communication), EDGE (Enhanced Data rates for GSM Evolution) and GPRS (General Packet Radio Service networks) services. The GANC thus utilises the A-interface towards the mobile services switching center MSC for 2G voice traffic (as defined in as defined in 3GPP TS 48.008) and the Gb-interface towards the serving GPRS support node SGSN when providing access to 2G/GPRS services (as defined in 3GPP TS 48.018). With the ongoing specification of generic access networks for third generation (3G) services, i.e. UMTS (Universal Mobile Telecommunications System) or WCDMA, the GANC provides access to 3G services (WCDMA/UMTS) and uses the Iu-cs interface towards the MSC and the Iu-ps interface towards the SGSN (as defined in 3GPP TS 25.410). The GANC selects the required mode of operation for each MS connected to it, and uses this mode as long as the MS is connected to the GANC. The existing mode of operation is called GAN A/Gb mode and the new mode of operation being specified is to be called GAN Iu mode.
The GANC (also called GAN cell) is identified differently for the mobile stations depending on which mode of operation is selected. These identifiers are used for example in idle mobility management procedures and when triggering either circuit-switched (CS) or packet-switched (PS) handover from the GSM or WCDMA (i.e. from GERAN or UTRAN) to the GAN. In GAN A/Gb mode, the GANC is identified as a GSM/GERAN cell using a cell global identity (CGI), an absolute radio frequency channel number (ARFCN) and base transceiver station identity code (BSIC) in the same way as GSM/GERAN cells are identified in GERAN and UTRAN. In GAN Iu mode, on the other hand, the GANC is identified as a WCDMA/UTRAN cell using a location area identity (LAI), 3G cell identity, a universal terrestrial radio access (UTRA) Absolute Radio Frequency Channel Number (UARFCN) and a primary scrambling code (PSC) in the same way as WCDMA/UTRAN cells are identified in GERAN and UTRAN. This also means that there is a difference in the way CS or PS handover (or relocation) procedures are triggered towards GAN A/Gb mode or towards GAN Iu mode. In GAN A/Gb mode, the conventional GSM/GERAN CS and PS handover procedures are used, as the GANC (or the GAN cell) is identified as a GSM/GERAN cell. In the same way, the CS or PS handover or relocation towards GAN in the GAN Iu mode uses the existing CS and PS handover procedures for WCDMA/UTRAN.
Handover from a PLMN access network to a generic access network is complicated by the lack of configured detailed information in the PLMN core network concerning the generic access network and the different access points used in the generic access network. Generic access networks preferably follow the principles of plug-and-play with access points to the generic access network being relatively small and easy to install, and registration of user equipment with the generic access network at different locations being possible independently of any PLMN coverage. This flexibility makes it near impossible to configure all PLMN's and all generic access networks with the data conventionally necessary for handover. For this reason, configuration of the PLMN and GAN elements is kept to a minimum. For example, in preparation for handover from a 3rd generation public mobile access network (also known as a UMTS access network or UTRAN) to a generic access network operating in GAN Iu mode, the radio network controller RNC of the UTRAN, when ordering the mobile station to make radio frequency measurements for neighbouring cells, includes a frequency number (the UARFCN) and scrambling code used on the identified frequency (the PSC) associated with the targeted generic access network controller (GANC). However, the corresponding frequency is not actually broadcast by the generic access network. Instead, the mobile station falsifies a measurement report indicating the highest signal level for the generic access network frequency number as a mechanism to try and initiate handover. In order to accomplish this, the mobile station must first have registered with the generic access network and received the UARFCN and scrambling code identifying the generic access network controller GANC. The mobile station will then recognise these parameters as originating from a generic access network when they are sent by the radio network controller.
In order to perform neighbouring cell measurements, a mobile station, or rather its user equipment, must monitor the cells indicated in the so called monitored set communicated by the RNC. Neighbouring cells may be located on the same frequency as the current cell and are in this case called intra-frequency cells. For the intra-frequency cells, only the PSC need be communicated by the RNC. The neighbouring cells may also use frequencies or even radio access technologies (RAT) that differ from that of the mobile station's current cell. For the inter-frequency cells, both the frequency and the PSC are communicated by the RNC. However, when the radio access technology used is wideband code division multiple access (WCDMA) the monitoring of frequencies is complicated by the fact that many mobile stations are unable to monitor different frequencies simultaneously. Specifically, when inter-frequency or inter-RAT neighbouring cells are included in the monitored set, the user equipment must be configured to enter so-called “compressed mode”, which means that the network must create time gaps for the user equipment, during which it can tune into another frequency and perform the requirement measurements. This obviously costs both time and capacity and also uses power, reducing the mobile station battery lifetime. For this reason, it is preferable to have a mobile station monitor the intra-frequency neighbouring cells, i.e. cells sharing the same frequency as the current cell.
When a generic access network cell is assigned a different frequency number, i.e. is an inter-frequency neighbouring cell, the cost is still greater, as a GAN enabled mobile will not actually perform frequency measurements on this frequency, but the network will still create time gaps. Moreover, mobile stations that are not GAN enabled will be constrained to search for a frequency that is not actually broadcast. Currently, there is no mechanism by means of which the generic access network controller can identify the frequency number used in a PLMN cell.
The mobile station must be registered with a GANC prior to handover being triggered and the WCDMA cell identity of the current serving cell used by the mobile station is communicated to the GAN in registration or registration update procedures. Conceivably, therefore, the GAN or GANC could be configured with an extensive database mapping WCDMA cell identities to WCDMA radio frequency numbers to enable a GANC to identify the UARFCN of the cell currently serving the mobile station and subsequently enable the GANC to identify itself to the mobile station as a cell on the same frequency, i.e. as in intra-frequency cell. However, notwithstanding the size required for such a database (a single WCDMA network can contain tens of thousands of cells), such a configuration would be impossible to maintain efficiently as it would need frequent updating to keep abreast of cell changes and network restructuring.
In the light of this difficulty there is a need for a mechanism whereby the GAN frequency number can be configured to correspond to that of the mobile stations current WCDMA cell, but which also retains the limited configuration or “plug-and-play” nature of GAN.